In the adaptive immune response, foreign antigens are recognized by receptor molecules on B lymphocytes (e.g., immunoglobulins) and T lymphocytes (e.g., T cell receptor or TCR). These foreign antigens are presented on the surface of cells as peptide fragments by specialized proteins, generically referred to as major histocompatibility complex (MHC) molecules. MHC molecules are encoded by multiple loci that are found as a linked cluster of genes that spans about 4 Mb. In mice, the MHC genes are found on chromosome 17, and for historical reasons are referred to as the histocompatibility 2 (H-2) genes. In humans, the genes are found on chromosome 6 and are called human leukocyte antigen (HLA) genes. The loci in mice and humans are polygenic; they include three highly polymorphic classes of MHC genes (class I, II and III) that exhibit similar organization in human and murine genomes (see FIG. 2 and FIG. 3, respectively).
MHC loci exhibit the highest polymorphism in the genome; some genes are represented by >300 alleles (e.g., human HLA-DRβ and human HLA-B). All class I and II MHC genes can present peptide fragments, but each gene expresses a protein with different binding characteristics, reflecting polymorphisms and allelic variants. Any given individual has a unique range of peptide fragments that can be presented on the cell surface to B and T cells in the course of an immune response.
Both humans and mice have class I MHC genes (see FIG. 2 and FIG. 3). In humans, the classical class I genes are termed HLA-A, HLA-B and HLA-C, whereas in mice they are H-2K, H-2D and H-2L. Class I molecules consist of two chains: a polymorphic α-chain (sometimes referred to as heavy chain) and a smaller chain called β2-microglobulin (also known as light chain), which is generally not polymorphic (FIG. 1). These two chains form a non-covalent heterodimer on the cell surface. The α-chain contains three domains (α1, α2 and α3). Exon 1 of the α-chain gene encodes the leader sequence, exons 2 and 3 encode the α1 and α2 domains, exon 4 encodes the α3 domain, exon 5 encodes the transmembrane domain, and exons 6 and 7 encode the cytoplasmic tail. The α-chain forms a peptide-binding cleft involving the α1 and α2 domains (which resemble Ig-like domains) followed by the α3 domain, which is similar to β2-microglobulin.
β2 microglobulin is a non-glycosylated 12 kDa protein; one of its functions is to stabilize the MHC class I α-chain. Unlike the α-chain, the β2 microglobulin does not span the membrane. The human β2 microglobulin locus is on chromosome 15, while the mouse locus is on chromosome 2. β2 microglobulin gene consists of 4 exons and 3 introns. Circulating forms of β2 microglobulin are present in the serum, urine, and other body fluids; thus, the non-covalently MHC I-associated β2 microglobulin can be exchanged with circulating β2 microglobulin under physiological conditions.
Class I MHC molecules are expressed on all nucleated cells, including tumor cells. They are expressed specifically on T and B lymphocytes, macrophages, dendritic cells and neutrophils, among other cells, and function to display peptide fragments (typically 8-10 amino acids in length) on the surface to CD8+ cytotoxic T lymphocytes (CTLs). CTLs are specialized to kill any cell that bears an MHC I-bound peptide recognized by its own membrane-bound TCR. When a cell displays peptides derived from cellular proteins not normally present (e.g., of viral, tumor, or other non-self origin), such peptides are recognized by CTLs, which become activated and kill the cell displaying the peptide.
Typically, presentation of normal (i.e., self) proteins in the context of MHC I molecules does not elicit CTL activation due to the tolerance mechanisms. However, in some diseases (e.g., cancer, autoimmune diseases) peptides derived from self-proteins become a target of the cellular component of the immune system, which results in destruction of cells presenting such peptides. Although there has been advancement in recognizing some self-derived antigens that elicit cellular immune response (e.g., antigens associated with various cancers), in order to improve identification of peptides recognized by human CTLs through MHC class I molecules there remains a need for both in vivo and in vitro systems that mimic aspects of the human cellular immune system. Systems that mimic the human cellular immune system can be used in identifying disease-associated antigens in order to develop human therapeutics, e.g., vaccines and other biologics. Systems for assessing antigen recognition in the context of the human immune system can assist in identifying therapeutically useful CTL populations (e.g., useful for studying and combatting human disease). Such systems can also assist in enhancing the activity of human CTL populations to more effectively combat infections and foreign antigen-bearing entities. Thus, there is a need for biological systems (e.g., genetically engineered animals) that can generate an immune system that displays components that mimic the function of human immune system.